This invention relates to dual-element current limiting electric fuses, and in particular to such fuses having circuit-completing triggers or connectors biased for circuit-opening movement.
Dual-element fuses of the afore-mentioned type normally have at least two spaced-apart electrical conductor members which are connected together by a trigger. Prior art triggers are connected to two fuse conductor members by heat softenable alloy, and are typically biased for circuit-opening movement out of contact with those fuse conductor members. Circuit opening movement is initiated when the alloy securing the trigger softens in response to a prolonged, relatively low overcurrent.
Prior art time delay fuses, such as that disclosed in U.S. Pat. No. 3,845,111, assigned to the same assignee as the present invention, have been provided with circuit-opening triggers which are characterized by multiple contact surfaces formed by interior wall members. Such a prior art trigger is typically made from an elongated strip of copper or other soft metal, which is formed into an S-shaped configuration. The S-shaped configuration provides first and second U-shaped cavities which are separated by an intermediate common wall. A first U-shaped cavity receives a free end of a first fuse conductor member and a second U-shaped cavity receives the free end of a spring member which biases the trigger for circuit-opening movement. The second U-shaped cavity is compressed to retain the spring in place by friction. Heat softenable alloy is applied to fill the remaining open areas of both cavities. The bottom-most surface of the S-shaped trigger is also secured to the free end of a second fuse conductor member by heat softenable alloy. Rather than apply discrete quantities of heat softenable alloy to the first and second U-shaped cavities and bottom surface of the trigger, commercial practice is to apply a large mass of heat softenable alloy which is typically poured over the entire trigger, after the trigger is positioned with respect to the first and second fuse conductor members and the spring bias member. While a greater quantity of heat softenable alloy is required for this construction, fabrication of the current limiting fuse is nevertheless simplified and provides a competitive cost advantage to the manufacturer of such fuses.
In response to a prolonged, relatively low, but potentially dangerous, overcurrent, the alloy softens allowing the trigger to be moved under the bias force of the spring member relative to the first and second fuse conductor members. Movement of the trigger accomplishes circuit opening by effectively withdrawing the free end of the first fuse conductor from the trigger cavity within which it is received. Resistance heating generated by the overcurrent, and which softens the alloy, continues to build even after circuit-opening movement of the trigger is initiated. In addition to resistance heating, extremely high temperature arcing is commonly experienced during circuit-opening movement of the trigger as it is separated from the first fuse conductor. As a result, the heat softenable alloy melts and, while in a fluid state, presents a risk of reconnecting the fuse conductor members after circuit clearing is initiated.
The legs of the S-shaped trigger defining the first U-shaped cavity of the trigger which receives the first fuse conductor member, are typically elongated, are closely spaced to minimize the amount of heat softenable alloy required to fill the cavity, and are positioned generally parallel to the first fuse conductor members. Because of the arrangement of the legs, it is sometimes difficult to achieve circuit-opening movement. As a result of the contact between the trigger and the first conductor member, frictional forces are experienced during retraction which can impair the circuit-opening capability of the fuse. Possible increases in the spring bias force to overcome these frictional forces are limited, in that unduly bulky springs cannot be accommodated within the confines of the current limiting fuse, and also, since increases in the spring force may jeopardize the mechanical securement of the spring to the trigger. Increases in spring force could also cause difficulties in maintaining the mechanical connection between the bias spring and the trigger because of "spring-back" of the trigger caused by the release of the compression engagement of the trigger about the spring loop. This "spring-back" phenomena tends to separate the adjacent folded portions of the trigger thereby lessening the capability of the trigger to engage the spring loop.